linux-kernel - Re: [RFC 1/3] lib: copy_{from,to}_user using gup & kmap

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Date:   Sat, 13 Jun 2020 13:51:26 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     afzal mohammed <afzal.mohd.ma@...il.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Linus Walleij <linus.walleij@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Nicolas Pitre <nico@...xnic.net>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
Subject: Re: [RFC 1/3] lib: copy_{from,to}_user using gup & kmap_atomic()

On Sat, Jun 13, 2020 at 05:34:32PM +0530, afzal mohammed wrote:

> Observation is that max. pages reaching copy_{from,to}_user() is 2,
> observed maximum of n (number of bytes) being 1 page size. i think C
> library cuts any size read, write to page size (if it exceeds) &
> invokes the system call. Max. pages reaching 2, happens when 'n'
> crosses page boundary, this has been observed w/ small size request
> as well w/ ones of exact page size (but not page aligned).
> 
> Even w/ dd of various size >4K, never is the number of pages required
> to be mapped going greater than 2 (even w/ 'dd' 'bs=1M')
> 
> i have a worry (don't know whether it is an unnecessary one): even
> if we improve performance w/ large copy sizes, it might end up in a
> sluggishness w.r.t user experience due to most (hence a high amount)
> of user copy calls being few bytes & there the penalty being higher.
> And benchmark would not be able to detect anything abnormal since
> usercopy are being tested on large sizes.
> 
> Quickly comparing boot-time on Beagle Bone White, boot time increases
> by only 4%, perhaps this worry is irrelevant, but just thought will
> put it across.

Do stat(2) of the same tmpfs file in a loop (on tmpfs, to eliminate
the filesystem playing silly buggers).  And I wouldn't expect anything
good there...